Do not undelegate previous events

I want to programmatically add event handlers to a view.

I tend to do this a lot (just using `jQuery.fn.on`), but my current specific use case is extending a some different views with common share functionality without resorting to inheritance. I can't just extend the view's existing `events` hash because of key collisions (same event and selector).

Currently `View#delegateEvents` calls `View#undelegateEvents` first, making it impossible to add additional handlers on top of existing ones. In this pull request, I've removed the call.

I know that this needs tests and documentation changes, but I wanted to gauge interest first before I commit more time.